A number of instruments are used in connection with measuring magnetic fields for the purpose of geophysical exploration, such as for locating regions likely to contain minerals, petroleum, water, natural gas, hazardous materials or other items or materials of interest. In general, geophysical exploration instruments for detecting magnetic fields can be considered in two categories: "passive" systems for measuring a magnetic potential field (near-zero frequency field), sometimes referred to as magnetometers, and active field instruments in which a transmitter emits electromagnetic radiation (periodic or other time-varying radiation) which travels through a portion of a geologic formation and is detected at a second location by a receiver or detector instrument.
Previous potential field instruments and active field instruments have typically been cumbersome to properly locate in the field, for taking desired data. In some cases, instruments are cumbersome because of their physical size or mass, often involving relatively large coils which, previously, were believed necessary in such instruments in order to attain the desired sensitivity. In instruments intended for borehole measurements, size is particularly important because of the strong relationship between borehole diameter and cost. Many instruments used for borehole measurement of magnetic field strength were relatively fragile, such as those based on a tuned ferrite rod.
Some previous instruments require special environments such as cryogenic environments or elaborate calibration or initialization procedures. Such previous instruments requiring cumbersome positioning or setup have, therefore, required a substantial investment in personnel time for each data-gathering section. In many exploration techniques, it is necessary to obtain data at a plurality of spaced-apart locations (such as a number of points along a line or a number of points in a grid). The instrument cost or personnel cost may make it infeasible to position a plurality of instruments simultaneously along the line or grid and thus one or few instruments are positioned and, after taking the desired data moved to another location along the line or grid. Thus, the personnel time requirements for taking data in an extensive region can be relatively high.
In many previous systems, separate instruments were needed for measuring a potential field (near-zero frequency field) and measuring an electromagnetic field (time-varying field), each instrument typically requiring its own cumbersome placement or setup procedures.
Generally, collecting data at more than one frequency can be desirable since location of targets is facilitated depending on the target size and depth below surface (lower frequencies being used for deeper or larger targets) and since the response for given target may be different at different frequencies for other reasons as well (e.g., target composition, and physical conditions of temperature, pressure and the like). Many previous instruments (including, many coil instruments, typically used for surface-based measurement and ferrite rod sensors used, e.g. for borehole measurements) were configured to sense only a single frequency or relatively small band of frequencies. Other instruments were configured to sense a plurality of different discrete frequencies (or relatively small frequency bands), but, typically, could sense only one frequency (or frequency band) at a time, (i.e. were incapable of sensing multiple frequencies substantially simultaneously and were incapable of sensing a frequency continuum or wide band width of frequencies). Thus, in many previous systems, in order to obtain data at a plurality of frequencies, it was necessary to take data at a first frequency during the first time period, reconfigure or substitute instruments and then take data at a second frequency at a later time. This characteristic of such instruments further contributed to relatively large personnel time requirements for geophysical exploration.
An additional reason for desirability of wide bandwidth sensitivity for geophysical instruments relates to collection of active field data. For data analysis purposes, the source (transmitter) waveform is often assumed to have an idealized form, such as the square wave signal depicted in FIG. 6A, defining a period T 612 and, thus, a frequency f=T.sup.-0.5. Although FIGS. 6A and 6B depict time-varying voltage signals, the source signal can also be a time-varying current signal, with appropriate changes in the receiving instrument. In order to detect a signal having frequency f, the (typically fixed) sample frequency f.sub.s must comply with the Nyquist criterion: EQU f.sub.s .ltoreq.2f.
Preferably, the sample frequency will be about 20 to 50 times the source signal frequency.
In practice it is typically not feasible to provide a square wave source signal, as depicted in FIG. 6A, and in most cases, the source signal will be more similar to the waveform depicted in FIG. 6B, having a finite falling or edge transition period t.sub.f which thus defines a transition frequency f.sub.t .apprxeq.0.35 (t.sub.f).sup.-0.5. Previous approaches had insufficient bandwidth to recover certain information, and typically performed data analysis by treating the source signal as if it were an idealized square wave signal (e.g. by ignoring the transition period). Such previous approaches thus failed to recover certain potentially useful information. In order to recover the integrity of the source signal of FIG. 6B, the detection instrument needs a system frequency response which can recover both f and f.sub.t. Typically, previous coil-based receivers have had insufficient bandwidth to detect both frequencies substantially simultaneously.
The above-noted relatively high personnel time requirements associated with previous instruments not only adds to the cost and delay in geophysical measurements but can affect the quality of the data and the time needed for data processing. This is because the earth's ambient magnetic field in a given location (which represents a background signal to the signal being measured) can vary significantly in the time frame typically required to complete a series of measurements using previous instrumentation. For example, if data is collected at a plurality of locations along a line, at a plurality of times during the day, it may require significant post-collection data processing to discriminate between changes in potential fields or electromagnetic fields arising from geophysical items of interest and those that result from the natural variation of the earth's magnetic field during the day. The same data discrimination problem is faced with regard to measurements at different frequencies which, as noted above, typically must be collected at different times. Moreover, even with sophisticated data processing techniques, it is not always possible to reliably discriminate between field changes arising from geophysical phenomena and those arising from variations in the earth's magnetic field.
In many previous systems, it was difficult, expensive and/or time-consuming to obtain vector information. For example, although coil-type detectors can have some degree of vector sensitivity, they have been generally difficult to deploy as two or three vector receivers.
Many previous systems had relatively high power requirements for the receivers or sensors. Such high power requirements added to the difficulty of obtaining field data since a large power source had to be provided in often remote locations.
Accordingly, it would be useful to provide a geophysical sensor for potential fields or active electromagnetic fields which are relatively less cumbersome and costly to provide, locate and setup, but are, preferably, non-fragile and relatively rugged, have good vector sensitivity, relatively low power requirements, have a relatively wide bandwidth, can measure both "passive" potential fields and active electromagnetic fields, preferably simultaneously, have a relatively high sensitivity and/or are feasible for collecting data at a plurality of locations and/or frequencies, or across a frequency range, substantially simultaneously.